Multi-radiator antennas are frequently used in for example cellular networks. Such multi-radiator antennas comprise a number of radiating antenna elements for example in the form of dipoles for sending or receiving signals, an antenna feeding network and an electrically conductive reflector. The antenna feeding network distributes the signal from a common coaxial connector to the radiators when the antenna is transmitting and combines the signals from the radiators and feeds them to the coaxial connector when receiving. A possible implementation of such a feeding network is shown in FIG. 1.
In such a network, if the splitters/combiners consist of one junction between e.g. 3 different 50 ohm lines, impedance match would not be maintained, and the impedance seen from each port would be 25 ohm instead of 50 ohm. Therefore the splitter/combiner usually also includes an impedance transformation circuit which maintains 50 ohm impedance at all ports.
A person skilled in the art would recognize that the feeding is fully reciprocal in the sense that transmission and reception can be treated in the same way, and to simplify the description of this invention only the transmission case is described below.
The antenna feeding network may comprise a plurality of coaxial lines being substantially air filled, each coaxial line comprising a central inner conductor at least partly surrounded by an outer conductor with insulating air in between. The coaxial lines may be parallel. The coaxial lines and the reflector may be formed integrally with each other in the sense that the outer conductors and the reflector are formed in one piece. The splitting may be done via crossover connections between inner conductors of adjacent coaxial lines.
Antenna feeding networks of the closed type are known, i.e. feeding networks where the outer conductor in each coaxial line forms a cavity around the central inner conductor, i.e. encircles or forms a closed loop around the central inner conductor as seen in a cross section perpendicular to the longitudinal direction of the coaxial line, see FIG. 2. One disadvantage with such a closed antenna feeding network is that it may be difficult to assemble the antenna, e.g. properly arranging the central inner conductors and associated components such as support means for holding the inner conductors and connection means between the inner conductors inside the outer conductors. Furthermore, if movable dielectric elements are provided between the outer and inner conductors to provide a phase shifting functionality, the positions of such dielectric elements are not easily adjustable due to the closed outer conductors.
Antenna feeding networks of the open type are also known, i.e. feeding networks where the outer conductors in at least some coaxial lines are provided with openings, and thus do not completely surround or encircle the inner conductors. One example of such a feeding network is disclosed in WO2005/101566 in which an antenna feeding network having coaxial lines with a longitudinally extending opening along one side of the outer conductor, see FIG. 3. The inner conductors are supported by dielectric support means. Pairs of adjacent inner conductors may be interconnected by cross-over elements, which are arranged in openings through the wall between the inner conductors. This feeding network solves some of the problems associated with the closed type feeding network, in particular it is easier to assemble since direct access to the interior of the coaxial lines is provided. On the other hand, the longitudinally extending openings makes the antenna less mechanically stable and unwanted backwardly directed radiation may occur. Such unwanted radiation may reduce the antenna performance in terms of e.g. back- or sidelobe suppression. In antennas having two cross-polarized channels, it may also reduce cross-polarisation isolation and also isolation between the two channels. All those antenna parameters may be important to the performance of e.g. a cellular network in terms of e.g. interference and fading reduction. The problem with unwanted radiation may be solved at least in part by additional components in the form of conductive covers to cover the cross-over elements. Using such covers add to the cost and complexity of the feeding network however.
US 2013/01355166 A1 discloses an antenna arrangement comprising an antenna feeding network including at least one antenna feeding line comprising a coaxial line having a central inner conductor and a surrounding outer conductor. The inner conductor is suspended inside the outer conductor with the help of dielectric support means. US 2013/0135166 A1 suggests to use a crossover element to connect two inner conductors of two adjacent coaxial lines. The crossover element is galvanically connected to the inner conductors by means of for example screws, soldering, gluing or a combination thereof, and thus a direct physical contact between the electrically conductive inner conductor and the crossover element is established. Where two conductors need to be connected, the wall between the two coaxial lines is partially or completely removed, and the crossover element is placed in the opening. The antenna arrangement according to US 2013/0135166 has the disadvantage that it may be difficult and time consuming to assemble or manufacture. A further disadvantage with this arrangement is that the mechanical connection formed by the screwed, glued or soldered connection between the lines may introduce passive intermodulation (PIM).
In order to preserve the characteristic impedance, the lines connecting to the crossover element include impedance matching structures. The substantially air filled coaxial lines may be provided with a dielectric element to provide a phase shifting arrangement. The phase shift is achieved by moving the dielectric element that is located between the inner conductor and the outer conductor of a coaxial line. If the dielectric element is moved in such a way that the outer conductor will be more filled with dielectric material, the phase shift will increase. WO2009/041896 discloses an antenna arrangement provided with an adjustable differential phase shifter using such a movable dielectric element.
The radiating element is typically a dipole. A dipole usually may consist of two radiating parts having an electrical length of approximately one quarter of a wavelength at the operating frequency and extending essentially in plane parallel with the antenna reflector, and positioned approximately at a distance equivalent to one quarter of a wavelength at the operating frequency. The radiating parts are fed in counter-phase. Such a feeding is achieved by using a balanced-unbalanced transformer, also called a balun. In a dipole, it is often convenient to also use the balun as a mechanical support of the two radiating parts. The balun is often also used as an impedance matching element.
The balun consists of a body part and a coupling element which can also be seen as a conductor positioned in the centre of a cylindrical hole in the body part. The balun coupling element is electrically connected at one end to one of the radiating elements, and at the other end to a feeding line inner conductor.
The body part is usually connected to feeding line outer conductor and to the antenna reflector.
The connection between the radiating element and one of the inner conductors may be achieved using for example a screw joint. Thus, direct contact between the electrically conductive coupling element of the radiating element and an electrically conductive portion of the inner conductor is established. Such an arrangement has the disadvantage that it may be difficult and time consuming to assemble or manufacture since a screwed connection may be difficult to achieve in the very limited space available inside the outer conductor. Also, the screw and the coupling element are often inserted from opposite sides of the antenna which makes assembly difficult. Another disadvantage with the screw joint is that it may introduce passive intermodulation (PIM). Due to the small dimensions of the coupling element of the radiating element, the screw joint also needs to be of small dimensions, which makes it particularly difficult to achieve a connection which is sufficiently firm to avoid PIM.